Integrated circuits (ICs) provide a cost effective compact solution for electrical circuits, typically including diodes, transistors, memory cells, logic circuits, and other circuitry with I/O terminals for interfacing the internal circuitry with external components on a printed circuit board (PCB). In addition to complex circuitry, many integrated circuits include internal capacitors and resistors, thereby mitigating the need for additional cost and circuit board space associated with providing external discrete components. In particular, resistors and conductive traces are sometimes formed during fabrication of one or more metallization layers to interconnect transistor terminals and other internal circuit components with one another and with externally accessible die pads. Printed circuit boards also include conductive traces or features for electrically interconnecting various components, such as discrete resistors, capacitors, diodes, and integrated circuits soldered to the board. For many applications, it may be desirable to embed resistors within metal traces in an integrated circuit metallization layer and/or on a printed circuit board to mitigate the need for discrete resistors thereby saving space and cost. Also, gradient resistance transmission lines are often desirable for infinite impedance matching, as well as gradient resistance features for current profile tapering.
Metallization layer processing typically involves creating geometrically patterned traces of copper or other suitable metal to create a patterned variation in resistance on a trace or feature. Embedded resistors can be created by adjusting the area (e.g., metal trace width) and/or length of a metallization layer structure, and gradient resistance transmission lines can also be created through geometric patterning for infinite impedance matching, current profile tapering or other applications. For example, the width or thickness of a metallization layer or printed circuit board trace may be narrowed to decrease the cross-sectional area, and the path of the trace may be extended through meandering or other length extending techniques to increase the resistance. Also, the trace width can be narrowed through tapering to provide a gradually increasing resistance. However, conventional metallization layer processing for integrated circuits and/or printed circuit boards may be limited in the ability to create integrated resistors, particularly gradient resistors, within cost and space restrictions. In particular, current techniques require either a change in geometry of the metallization layer trace and/or deposition of other materials that have a higher or lower resistance per square.
Geometrically varying feature shapes and lengths to create lower resistance features may not be practical in certain situations because of space or area limitations, while cost, complexity and fabrication tolerances may limit the feasibility of creating higher resistances and/or gradient resistors. These conventional techniques may be unsuitable for high frequency structures where the geometry of the metallization is directly linked to high frequency performance, or for gradient current profile tapering. The other approach involves depositing two or more materials onto a PCB or integrated circuit metallization layer, which have different impedance per unit area values (ohms per square). However, this technique increases cost and fabrication time, and still only allows for a discrete number of degrees of freedom in creating different resistances and profiled resistances. As a result, this approach is often used in combination with geometric patterning, which may be contrary to size restrictions for a given design. Thus, conventional techniques involving variations to trace geometry are limited by available space for low resistances and by process restrictions for minimum trace widths and minimum feature sizes in creating high resistances.